In order to increase the efficiency and decrease the manufacturing cost of photovoltaic (PV) cells, significant efforts have been made to develop rear-contact solar cells in which both the positive and negative polarity contacts of the solar cell are accessible from the rear, or non-light-incident, side of the cell. Compared to traditional front-contact solar cells, rear-contact solar cells typically have less or in some cases no metal coverage of the cell's front surface. This circumvents the tradeoff that occurs in front-contact cells between the conductance of metallic front electrodes and their coverage (or shadowing) of the cell's light-incident side, leading to better optical in-coupling, lower resistive power loss, and higher conversion efficiency. Examples of rear-contact solar cells may be found in U.S. Pat. Nos. 3,903,427, 3,903,428, 4,927,770, 5,053,083, 7,276,724, and US patent applications US2009/0314346, US2010/0139746, and US2009/0256254. A thorough review of silicon-based rear-contact solar cell technology may be found in Prog. Photovolt: Res. Appl. 2006; 14:107-123.
In addition to providing higher efficiency, there are at least two other ways in which the incorporation of rear-contact solar cells can simplify and reduce the cost of manufacturing PV modules. First, in a rear-contact PV module production line, it may be possible to replace the tabbing and stringing operation required in front-contact PV modules with a simple placement step in which rear-contact cells are directly connected to an electrically functional interconnect backsheet just prior to module lamination. This can help enhance the overall throughput of the production line. Second, for silicon-based PV cells, rear-contact PV modules are typically better-suited than front-contact PV modules to the incorporation of thin, large cells because front-contact silicon cells may develop a large coefficient of thermal expansion (CTE) mismatch stress when thick current-collecting tabs are soldered on to them. This CTE mismatch stress and associated cell breakage is particularly problematic if the cells are thinner than about 200 microns or larger than about 156 millimeters on a side. By contrast, the need for thick metallic conductors is significantly reduced in rear-contact solar cells because the output current is typically distributed across the back surface of the cell. This enables thinner, wider metallic conductors to be attached to the back of rear-contact cells with lower ohmic power loss and reduced breakage from CTE mismatch stress.
At present, however, several factors related to the difficulty in interconnecting rear-contact solar cells have limited their widespread implementation. For example, in many rear-contact solar cell architectures, it is desirable to have a contact size and spacing on the order of a few millimeters on the cell's rear surface, while the interconnected assembly of rear-contact solar cells is generally 1 m2 or larger in a finished PV module. It is very difficult to fabricate a single circuit or device that can accommodate both of these dimensional requirements with high yield and low cost. Large-area “conductive backsheets” typically utilize a layer of interdigitated positive and negative electrodes disposed just below the rear-contact solar cells (see, for example, US Patent App. No. 2010/0012172). In many cases, the production cost of these conductive backsheets is so high that their use in PV modules becomes impractical. The high cost can be attributed in part to the relative lack of availability of screen printing and etching equipment that can handle 0.5-2 m wide rolls of material, and in part to the stringent material quality required to ensure that circuit patterns this large retain their dimensional stability to within a few millimeters.
In addition, achieving sufficient long-term reliability from rear-contact solar modules incorporating large-area conductive backsheets has been a challenge. For example, these devices may be prone to electrical shorting during fabrication and/or long-term outdoor exposure if an electrode of one polarity on the conductive backsheet touches an electrode of the opposite polarity on a rear-contact solar cell. Furthermore, in some cases mechanical stress arising from CTE mismatch between the rear-contact solar cells and the conductive backsheet may be amplified due to the fact that the conductive backsheet is generally much larger than the solar cells. Expansion and contraction of the conductive backsheet relative to the solar cells and the front glass cover sheet can cause large shear stresses to form on materials such as solder or electrically conductive adhesive that are used to make electrical connections between the solar cells and the conductive backsheet. Over time this may lead to fatigue and/or failure of these electrical connections in the field. Therefore, there exists a need for an improved interconnect for rear-contact solar cells.